The Mesozoic Era began some 200 million years ago and ended some 60
million years ago. Although many other animals lived during that era,
the dinosaurs were the dominant forms of animal life on land. The 140
million years of the Mesozoic are divided into geologic periods named
Triassic (the oldest), Jurassic, and Cretaceous (the most recent).
Continental deposits representing each of these periods have been found
in all parts of the world and on all continents. Dinosaur bones have
been found in these deposits—even in such far-away places as Australia
and the southern tip of South America. Only Jurassic dinosaurs have been
found at Dinosaur National Monument.
[Illustration: _Coelophysis_—SMALL TRIASSIC DINOSAURS, FORERUNNERS
OF THE HUGE DINOSAURS OF JURASSIC PERIOD. (DRAWN BY MARGARET M.
COLBERT. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)]
The oldest known dinosaurs are found in rocks of the Triassic Period.
The smaller of these were chicken-size and the largest were about as big
as kangaroos. All of these Triassic dinosaurs were two-footed. They can
be divided into flesh-eaters and plant-eaters, although none are
believed to have been particularly specialized in their food habits. In
general the flesh-eaters were small, agile, and had sharp teeth for
seizing and overpowering active prey. The plant-eaters were larger with
rather long front legs and small blunt teeth suited only to cropping
vegetation. These plant-eaters are believed to be the Triassic ancestors
of the giant marsh-dwelling dinosaurs of the Jurassic and Cretaceous
Periods.
A greater variety of dinosaurs lived during the Jurassic Period than in
the Triassic. Both two- and four-footed types were present. The
flesh-eaters remained two-footed but increased in size. _Antrodemus_,
perhaps the best known, was much bigger than a kangaroo. The larger
plant-eaters weighed from 30 to 40 tons and all were four-footed. The
largest land animals, they lived on dry land and in the swamps that
formed an important part of the Jurassic landscape. The first of the
armored plant-eating dinosaurs, _Stegosaurus_, inhabited the dry plains.
There were also some smaller, kangaroo-size plant-eaters that were
two-footed.
A wide variety of dinosaur fossils has been found in the rocks of the
Cretaceous Period, the last of the Mesozoic Era. The huge swamp dwellers
still thrived. The flesh-eaters had evolved much larger types and
included 40-foot _Tyrannosaurus_, the largest that ever lived. All the
flesh-eaters walked on their hind legs as did their predecessors of the
Jurassic and Triassic Periods.
New and interesting dinosaurs were present among the flesh-eaters.
Horned forms, somewhat similar to the rhinoceros but much larger, were
common. Also common were the turtle-like ankylosaurs. Perhaps the oddest
and most interesting dinosaurs of the Cretaceous were the two-footed
hadrosaurs. These excellent swimmers had weird head shapes with
complicated skull passages and openings. They were a very successful
group and at least 15 different kinds are known from the Cretaceous
rocks of North America.